The importance of internal burned gas residual to engine combustion quality has long been recognized. Historically, the motivation for developing residual estimation methods comes from the fact that it is needed as input to a heat release rate analysis. More recently, it has been recognized that the use of variable valve actuation (VVA) to control and maximize internal dilution may enable the elimination of external EGR systems, along with significant fuel economy and NOx control improvement. More recently the role of internal dilution in the control of advanced-mode combustion systems, such as homogenous charge compression ignition (HCCI), has also been explored. Due to the above factors, there has been a surge of interest in methods of measuring or estimating engine residuals during engine tests.
The physical process of residual generation is complex. During the gas exchange process pressure and velocity pulsations are generated in the intake and exhaust manifolds due to fluid inertia and wave action. These pulsations strongly affect the gas flows through the engine valves that determine the residual content of the trapped charge. Because of the complexity of the process, various experimental techniques have been applied to measure residuals in engines. These experimental techniques can be broadly classified into a) optical, and b) gas-sampling methods. These all require a complex experimental apparatus and are time consuming to perform.
In view of the difficulty of empirical measurement, and in view of recent advances in computer engine cycle simulation, there has been significant effort toward modeling the residual generation process. In one approach, a highly detailed simulation model of the engine and the manifold system is constructed and carefully calibrated against engine test data (e.g., airflow, temperatures and combustion rates) over the entire operating range of interest. The main disadvantage is that the creation and calibration of a sufficiently accurate model is a difficult task, so a substantial time investment by an engineer highly skilled and knowledgeable in the field of engine simulation is required.
A second approach for a detailed process simulation may be referred to as the “port-pressure method”. A very simple, fast-executing simulation model of a single engine cylinder (rather than multiple cylinders), without intake and exhaust manifolds, is constructed. Rather than attempting to accurately model the admittedly complex intake and exhaust port pressure dynamics, pressure transducers are installed in the intake and exhaust ports of a dynamometer test engine to measure crank-angle-resolved intake and exhaust port pressure data. These are then used as inputs to the simulation and imposed as boundary conditions, while cylinder pressure data is used to derive the combustion rate inputs. While this method is coming into fairly common use, a disadvantage is that the instrumentation required on a multi-cylinder test engine is somewhat elaborate, costly, and time consuming. Also, the results are sensitive to model inputs for valve train compliance (e.g., effective valve lash) and port flow coefficients for both flow directions at low valve lift, both of which are difficult to accurately measure.
There is therefore a need for a system and method for estimating residual burned gas fraction that minimizes or eliminates one or more of the problems set forth above.